1. Field of the Invention
This invention relates to an electro-acoustic transducer advantageously applied to a small-sized earphone unit, headphone unit or a small-sized microphone unit.
2. Description of the Prior Art
As the electro-acoustic transducer of the type in which a housing enclosing an electro-acoustic transducer device is inserted into and attached within the ear's concha, when the transducer is in use, there is proposed in the art a so-called inner ear type headphone unit.
Such headphone apparatus is shown in FIG. 1 and includes a housing 102 enclosing a speaker unit which is the electro-acoustic device. This housing 102 is formed with a size such that it can be introduced into the ear's concha and has an open sound-radiating surface of the speaker unit 101. A connecting cord 103 supplying acoustic signals to the speaker unit 101 has its ends 104 connected as by soldering to input terminals 105 of the speaker unit 101. The connecting cord 103 is fed out of the housing 102 to the outside by way of a cord extraction opening 106.
An ear pad ring 107 is provided on the portion of the housing 102 facing to the ear's concha and on the perimeter of the front side of the speaker unit 101. This ear pad ring 107 is engaged with a rib 102a provided on the perimeter of the open portion of the housing 102, and is attached to this portion by auxiliary securing means, such as adhesive.
The housing 102 is formed of a relative hard material in consideration that it has to hold the speaker unit 101 in position and for providing certain acoustic effects. On the other hand, the ear pad ring 107 is formed of a relatively soft material for providing good contact feel with the inner wall of the ear's concha and insertion of the ring into the ear's conchae of different sizes.
In order that the portion of the connection cord 103 extracted through the extraction opening 106 may be protected from accidents such as cord breakage caused by repeated cord flexure, a cylindrical cord strain-relief bushing 108 is provided on the housing 102 extending from the cord extraction opening 106. The cord bushing 108 is formed of a relatively soft material, similar to the ear pad ring 107, and is provided with peripheral slots 109 to allow the bushing 108 to flex easily. The portion of the connection cord 103 extracted through the cord extraction opening 106 may be bent smoothly as it is encased within the cord bushing 108 when a stress is applied from outside to prevent breakage of the cord at the edge of the cord extraction opening 106.
In the operation of the above described headphone unit, when the housing 102 is introduced into the ear's concha, the ear pad ring 107 is deformed under a pressure applied from the inner wall of the ear's concha. The ear pad ring 107 is adapted to close the ear's concha completely by abutting on the inner wall of the ear's concha by its own resiliency. With the ear's concha thus closed completely, the sound may be reproduced by the speaker unit 101 with optimum electro-acoustic characteristics.
In will be noted that, since the size of the ear's conchae will differ from one person to another, it may occur that the above described headphone unit cannot be introduced into the ear's concha, or the ear's concha cannot be closed satisfactorily by the ear pad ring 107, with the result that optimum acoustic characteristics are not obtained.
That is, with the outside diameter d.sub.1 of the ear pad ring 107, the ear pad ring cannot be deformed to an outside diameter d.sub.0 of the rib 102a. Therefore, when the ear's concha has an inside diameter less than the outside diameter d.sub.0 of the rib 102a, the housing 102 cannot be introduced into the ear's concha. On the other hand, when an ear's concha has the inside diameter larger than the outside diameter of the ear pad ring 107, a void or gap is formed between the ear pad ring 107 and the inner wall of the ear's concha, so that the ear's concha is not closed completely.
When the ear's concha is not closed completely in this manner, the output is lowered especially in the lower frequency range of the acoustic characteristics, as shown at C in FIG. 9, with result that optimum electro-acoustic characteristics cannot be realized.
It is noted that the acoustic characteristics shown at B in FIG. 9 are those obtained when the ear's concha is closed as normally in the above described conventional headphone unit.
For maintaining the optimum acoustic characteristics as described above, it is necessary that the ear pad ring 107 be formed of a softer material and with an increased thickness as that it may be deformed radially more easily and with a larger amount of deformation. However, when the housing 102 is mounted to or detached from the ear's concha, a stress is applied to the ear pad ring 107 tending to displace the ear pad ring 107 away from the housing 102 in the direction shown by arrow F FIG. 1 under the force of friction with the inner wall of the ear's concha. When the ear pad ring 107 is formed of a soft and pliable material, the ear pad ring is deformed easily under such stress. When the ear pad ring 107 is deformed under the stress in this manner, both the aforementioned engaging and auxiliary securing means tend to be loosened so that the ear pad ring may be readily detached from the rib 102a.
In short, when the material of the ear pad ring is formed of a softer material, the size range of the ear's conchae into which a given ear pad ring can be introduced is enhanced, while the contact feel of the apparatus with the ear's concha is improved. However, when the ear pad ring is formed of a softer material, the ear pad ring is more likely to be detached from the housing, so that of the earphone unit durability is not assured. For this reason, the desired durability may only be assured at the sacrifice to some extent of the aforementioned acoustic characteristics and the use or contact feel.
The same inconvenience as felt with the ear pad ring arises with the cord bushing 108. That is, when the cord bushing is formed of a softer material, the connection cord 103 may be protected more reliably, however, the cord is more likely to be detached from the housing 102.
In our copending Japanese Laid-open Patent Publication No. 23299/1987, there is proposed an electro-acoustic transducer in which the housing enclosing the speaker unit, ear pad ring and the cord bush are molded integrally from the same material for facilitating the assembling operation and improving the durability. However, in this electro-acoustic transducer, since the portion contacting the ear's concha and the housing adapted to hold the speaker unit are formed of the same material, the material suitable for both the contacting portion and the housing cannot be used, such that the contacting portion and the housing need be formed of a material having only moderate hardness.